where there is a line series (periods) representing lod score vs
chromosomal position for a series a statistical test spanning the
chromosome. Peaks in the lod score indicate position of a gene.
Confidence in position is designated with equal signs (a 2 lod interval
surrounding a peak). It's not hard to use three line commands in a
graph tw, to generate the plot,

But each plot takes up one serset (3 for the above command). I was
wondering how to collapse the two plots (line baseline gene1) (line
baseline gene2) into (one). Doesn't seem possible, because graph tw
(y1 y2 x) is not applicable here. Am I missing something easy?